It is known that optical cables in which the core comprises a member of cylindrical section and which is provided, on the outer surface thereof, with grooves extending in the form of a helix along said surface are divided into two groups.
The first group comprises cables in which the grooves present on the outer surface of the member of cylindrical section extend in the form of a closed helix whereas the other group comprises cables having grooves in the form of an open helix.
By the term "closed helix", it is herein meant that all turns of the helix are oriented in the same way and all have an S-shaped or Z-shaped configuration. By the term "open helix", it is herein meant that adjacent portions of the helix extend in two directions, and therefore, that the helix is formed with S-shaped turns or Z-shaped turns which are disposed alternately with each other.
It is known that for equal diameter sizes of the core and equal the pitch, shape and sizes of the grooves, the cables in which said grooves have the form of a closed helix permit a greater excess length of the optical fibers in said grooves, that is, a greater excess with respect to the length of the core axis, as compared with the cables in which the grooves have the form of an open helix. The reason for this difference will be explained hereinafter.
The maximum permissible excess length of an optical fiber in a helix-shaped groove is directly proportional to the difference between the length of a fiber which engages, throughout its length, the surface closing the outside of said groove (herein also referred to as "line of maximum groove extension") and the length of a fiber which engages the bottom line of the groove throughout its length.
In an open-helix groove, there are a number of portions in which the bottom line thereof and the line of the maximum groove extension are parallel to the cable axis, and this occurs at the junction portions betwene each S-shaped turn and each Z-shaped turn.
In these junction portions, the length of the bottom line of the groove is equal to the length of the maximum extension line in said groove.
On the contrary, in a closed-helix groove, where the turns are oriented in the same way, there is no portion in which the optical fiber axis, the bottom line of the groove and the maximum extension line of the groove are parallel to the cable axis, and therefore, at all axial positions the maximum extension line of the groove is always longer than the bottom line of the same.
As a result, the difference in length between the maximum extension line and the bottom line in an open-helix groove is shorter than the difference between them in a closed-helix groove, and therefore, in the latter, the maximum permissible excess length of a fiber which can be given to an optical fiber is inherently greater than that permitted in the open-helix grooves.
The advantage of permitting a greater excess length of the optical fibers of cables having a core provided with closed-helix grooves in which the optical fibers are loosely housed, as compared with cables of the same type in which the grooves for loosely housing the optical fibers having an open-helix configuration, is practically reduced to zero due to the impossibility of executing joints in which the outer diameters are the same as the outer diameters of the cables without running the risk of damaging the optical fibers and when using the usual butt welding techniques between the optical cable fibers so as to practically recreate the same structure in the joint as is in the core. For example, as disclosed in U.S. Pat. No. 4,842,438.
In fact, as stated in said Patent, by interposing a cylindrical body provided with the same grooves as those of the cable cores to be united, and therefore, in the case of cables having a core provided with closed-helix grooves necessarily has closed-helix grooves oriented in the same way as those of the cable cores, it is impossible, once the junctions between the end portions of the optical cable fibers have been carried out, to introduce said junction portion into the grooves of the cylindrical body connecting said cable cores without exceeding the maximum excess length value permissible to an optical fiber. As is known, if the excess length exceeds the permissible length, there is a risk of microbending of the optical fibers in the groove receiving it which is deleterious.
This is due to the fact that by using a portion in the joint area provided with closed-helix grooves like those of the cable core, the portion of optical fibers designed to carry out the continuity between the optical fibers in the joint, first of all, must necessarily be arranged in the form of a closed helix about the axis of the joint. Secondly, said portion must be of greater length than that of the maximum extension line of the groove in order to enable the welding devices to carry out the welding between the optical cable fibers because of the bulkiness of said devices.
For such reason, in the case of joints between optical cables of the type in which the cores have the grooves designed to receive the optical fibers in the form of a closed helix, complicated structures of joints are proposed, such as the one disclosed in the Japanese Patent Application No. 55-95917.
The use of complicated structures for joints of optical cables, the cores of which are provided with closed-helix grooves, gives rise to important difficulties during the laying of these cables, particularly, when they are used in a submarine environment. This is due to the radial bulkiness of the joints present therein which causes difficulties in passing the joints through the pulleys of the devices designed to carry out the laying.